Merge rust-bitcoin#2925: Fix fuzz script

31a2324 Fix fuzz script (Tobin C. Harding)

Pull request description:

  Currently the `fuzz.sh` script fails with "unbound variable" if called without any arguments, this has gone undetected since we added `set -euox pipefail` because in CI we always call it with an argument.

  Use chatGPT to fix the bug.

  Fix: rust-bitcoin#2924

ACKs for top commit:
  apoelstra:
    ACK 31a2324

Tree-SHA512: 28e018bafd02c4034863c84e3b491c83962379d41bddeae184c4e35b889cb3e5aed9605b01749a97c60e3090525db52786574e487ff20cbf61fbdc8eaa8ab5a0

fuzz/fuzz.sh

@@ -10,7 +10,7 @@ source "$REPO_DIR/fuzz/fuzz-util.sh"
 # Check that input files are correct Windows file names
 checkWindowsFiles
 
-if [ "$1" == "" ]; then
+if [ -z "${1:-}" ]; then
   targetFiles="$(listTargetFiles)"
 else
   targetFiles=fuzz_targets/"$1".rs